Add the minimal amount of smarts necessary to instcombine of shufflevectors to recognize

Add the minimal amount of smarts necessary to instcombine of shufflevectors to recognize
patterns generated by clang for transpose of a matrix in generic vectors.  This is made
of two parts:

1) Propagating vector extracts of hi/lo half into their users
2) Recognizing an insertion of even elements followed by the odd elements as an unpack.

Testcase to come, but this shrinks the # of shuffle instructions generated on x86 from ~40 to the minimal 8.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110734 91177308-0d34-0410-b5e6-96231b3b80d8